Why Is Contaminated Soil Not Simply Incinerated During Desorption?

In thermal desorption, contaminated soil is not incinerated but heated in the absence of oxygen. This crucial difference prevents the formation of hazardous combustion products such as dioxins and makes it possible to remove contaminants in a targeted manner without destroying the mineral material. The following questions explain how the process works, which contaminants it can treat, and when it is used.

What happens to contaminants during thermal desorption?

In thermal desorption, contaminants are released from the soil material by heat and converted into a gaseous state. This occurs at temperatures of up to approximately 600 °C in a rotary kiln, depending on the material and contaminant load. The released pollutant gases are then treated and destroyed in a multi-stage exhaust gas cleaning system before leaving the plant.

The process takes place in two separate phases. In the rotary kiln, the organic contaminants detach from the soil material and are drawn off as raw gas. This raw gas then passes through thermal afterburning, flue gas scrubbing, and an activated carbon filter. Only after this complete treatment chain are the exhaust gases discharged in a controlled manner. The cleaned soil leaves the kiln as mineral material that can be reused or properly disposed of.

Why is the exclusion of oxygen so critical when heating soil?

The exclusion of oxygen prevents organic contaminants from combusting in the rotary kiln and thereby forming new, sometimes even more hazardous compounds. Without oxygen, no uncontrolled combustion process can take place. Contaminants are instead thermally desorbed — i.e., vaporized — and can be captured and destroyed in a targeted manner in the exhaust gas cleaning system.

This point is particularly relevant for contaminants such as polychlorinated biphenyls (PCB) or dioxins and furans (PCDD/PCDF). If oxygen-containing gas were to flow through the kiln, new dioxins could be formed at certain temperatures — an effect known in the specialist community as de novo synthesis. The exclusion of oxygen eliminates this reaction pathway and is what makes the process both safe and approvable. At the same time, the mineral base material of the soil is largely preserved because it is not oxidized.

Which contaminants can thermal desorption treat?

Thermal desorption is suitable for a broad spectrum of organic and inorganic contaminants. Treatable substances include, among others, mineral oil hydrocarbons (TPH), BTEX compounds, phenols, polycyclic aromatic hydrocarbons (PAH), PCB, PFAS and PFOS, dioxins and furans (PCDD/PCDF), cyanides, nitroaromatics, and numerous other chlorinated hydrocarbon compounds. Mercury (Hg) can also be treated in the process.

The wide range is explained by the operating principle: contaminants that can be volatilized or decomposed at the temperatures used are generally suitable for the process. For complex contaminant mixtures, such as those commonly found at former industrial sites, the precise control of the treatment temperature enables targeted adaptation to the specific contamination situation. Soils with multiple contaminations — meaning simultaneous pollution by different contaminants — can thus be remediated in a single treatment step.

What is the difference between desorption and incineration of soil?

The key difference lies in the oxygen content and the objective of the process. In incineration, organic material is oxidized and destroyed by the introduction of oxygen. In thermal desorption, oxygen is absent: contaminants are vaporized and treated separately, while the mineral soil material is preserved.

This has practical consequences for the treated soil. After incineration, essentially only ash remains — the original soil structure is destroyed. After thermal desorption, a cleaned mineral material is produced that, depending on its quality, can be reused as a construction material or backfill. This conserves resources and reduces the volume of waste that must be landfilled.

Another difference concerns emissions. Incineration plants operate at significantly higher temperatures and generate larger volumes of flue gases. Thermal desorption produces a more concentrated raw gas that can be treated more efficiently in the downstream exhaust gas cleaning system. More details on the technical fundamentals of thermal soil treatment are explained in the process description.

For which remediation projects is thermal desorption suitable?

Thermal desorption is particularly well suited for sites with complex organic contaminations where other treatment methods reach their limits. Typical areas of application include decommissioned gasworks, former industrial sites, military properties, and sites of chemical plants.

The following project types are commonly encountered:

Remediation of gasworks sites with PAH and BTEX contamination
Disposal of soil and construction rubble from demolition projects with PCB or asbestos cement contamination
Treatment of dredged material from contaminated bodies of water or harbor basins
Revitalization of industrial brownfields with multiple contaminations
Disposal of process residues such as slags, ashes, and sludges from industrial plants

What matters for suitability is less the origin of the material than its contaminant composition. Thorough analysis prior to remediation is therefore essential. As part of a comprehensive material flow management approach, it is possible to determine during the planning phase which material streams are suitable for thermal treatment and which must be disposed of by other means.

How is it ensured that treated soil is truly free of contaminants?

Quality assurance in thermal desorption takes place on several levels: through process control in the rotary kiln, through monitoring of the exhaust gas cleaning system, and through analytical testing of the treated material. The material is only released once the analytical values comply with the statutory limit values.

In concrete terms, this means: temperature, residence time, and oxygen content in the rotary kiln are continuously monitored and regulated. The multi-stage exhaust gas cleaning system — comprising dust removal, thermal afterburning, flue gas scrubbing, and an activated carbon filter — is also continuously monitored. Emission limit values under the Federal Immission Control Act must be complied with and are subject to regulatory oversight.

After treatment, the cleaned material is sampled and analyzed for relevant contaminants. The results determine its further use or disposal. This systematic approach is part of a certified quality management system in accordance with ISO 9001, which prescribes binding processes for every treatment step.

How ARE Deutzen GmbH helps with thermal soil treatment

ARE Deutzen GmbH has been operating one of the most powerful thermal soil treatment plants in Germany since 1994. The company accepts contaminated soils, construction rubble, and mineral waste and ensures legally compliant, harmless treatment that meets all statutory requirements. Customers benefit from more than 30 years of experience with complex contamination situations and over 2.5 million tonnes of treated material.

The range of services specifically includes:

Thermal desorption in a rotary kiln at temperatures of up to approximately 600 °C, depending on the material and contaminant load, under exclusion of oxygen
Treatment of a broad spectrum of contaminants, from TPH and PAH to PFAS, PCB, and dioxins
Mechanical-physical pre-treatment and interim storage prior to thermal treatment
Complete material flow management for various waste types and project situations
Analytical support and documentation for regulatory verification purposes
Certified waste management company in accordance with the Circular Economy Act

If you are planning a remediation project or need to dispose of contaminated material, contact ARE Deutzen directly. Through the contact persons on the website, you will receive an expert initial assessment of your specific case.